To optimize the design and fabricate the X- ray lens, the method of rotating coordinate system is used to establish the equation of the tubes. Some mathematical transformations and formulae are used instead of iterative method to find out the intersection of the X- ray and the wall of the capillary, so that X- ray′ s trajectory can be traced. The transmission rule of the X- ray propagation in the X- ray lens can be got, after tracing a large number of X- ray beams. A program based on ray- tracing method is designed to simulate the transmission efficiency of the X- ray lens and the light spot gained from it. Two parallel lenses are tested to verify the rationalization of the model. As to the transmission efficiency, the experimental result is 13.0% and the simulation result is 14.2% when the energy of the photon is 8.05 keV. And there is a notable similarity between optical and simulated images of the light spot. The results show that the mathematical model is reasonable, and it has the potential of designing and fabricating X-ray lenses.